Conventionally, in an image recording medium and a recording-reproducing apparatus using the same, a video tape, a digital versatile disk, a hard disk, and the like have mainly been employed. Also, in a music recording medium and a recording-reproducing apparatus using the same, a magnetic tape, a writable compact disk, a flash memory (floating-gate transistors), and the like have mainly been employed. Further, a floppy disk, a hard disk, a digital versatile disk, a writable compact disk, a flash memory, a ferroelectric memory, and the like have been employed for storing data in a computer or the like.
A memory apparatus of a writable type that can maintain stored data after power is shut-off is classified according to a writing method into an apparatus of a type in which a recording medium is scanned or rotated, such as a magnetic recording apparatus, a magneto-optic recording apparatus, and a phase change recording apparatus, and an apparatus of a matrix type in which mechanical scanning or rotation is not required, such as a semiconductor memory and a ferroelectric memory.
The following devices have been known, but they have not been practical for use. Japanese Patent Laid-Open Publication No. Hei 6-28841 discloses a memory apparatus which utilizes atomic or molecular electrophoresis or an electrochemical reaction in an ionic conductive material (an electrolyte). U.S. Pat. No. 3,271,591 discloses a phase change memory which utilizes the characteristics of phase change of chalcogenide caused by a temperature change. Further, in U.S. Pat. No. 5,363,329, a device which utilizes a metal ion precipitating phenomenon in chalcogenide serving as an ionic conductive material has been proposed. However, in the above patent, the ionic migration principle is misidentified or misdescribed as electromigration.
An electronic device which was recognized to utilize electromigration, or the basis of the present invention, includes the electronic device disclosed in Japanese Patent Laid-Open Publication No. Hei 08-293585 and that disclosed in Japanese Patent Laid-Open Publication No. 2001-267513. Although the former device has little practical use, the latter seems to be practical.
However, in a recording-reproducing apparatus of a type which requires scanning or rotation of the above described recording mediums, a mechanically movable portion must be provided, and the limit of size and weight reduction has already been reached. In addition, the mechanical shock resistance thereof is low, and the writing and reading speed is significantly slowed by the time required to move to the recording position on the medium.
A semiconductor memory apparatus such as a flush memory requires complicated production steps, and the cost is high. Therefore, a semiconductor memory apparatus having a large storage capacity comparable to that of a magnetic recording apparatus has not been widely used in practice. Further, a ferroelectric memory having a large storage capacity comparable to that of a magnetic recording apparatus is unlikely to be implemented due to the relatively complicated structure thereof.
A memory apparatus utilizing an electrochemical reaction of an electrolyte has a slow reading-writing speed since a response delay with respect to a high frequency wave or a short-time pulse (caused by a charge accumulation time due to an electric capacity and molecular polarization) always takes place in an ionic conductive material. In addition, the choice of the material which contacts with a chemically active ionic conductive material is limited. Particularly, in order to ensure long-term reliability, a noble metal, a refractory metal, or the like must be employed as an electrode material. Despite this, the long-term durability of the device is still inadequate.
The above Japanese Patent Laid-Open Publication No. Hei 08-293585 discloses a nonvolatile memory device utilizing so-called electromigration. In this case, an alloy composition is biased through supplying an electric current to the alloy serving as an electronic conductor. However, even if the materials, the devices, and the recording methods disclosed in the above publication are fully utilized, the nonvolatile memory device is still far from practical use. The wiring structure disclosed in the above publication is formed of an aluminum alloy and a tungsten electrode (a plug), and is a wiring structure commonly employed in an LSI. Conventionally, the detailed reliability tests for this wiring structure have been repeatedly performed at various temperatures. In this technical field, it is well known that the resistance increase caused by electromigration in the LSI wiring does not take place uniformly, but takes place highly non-uniformly. Therefore, the technology disclosed in the above publication cannot be employed for simultaneous change of a large number of devices with high reproducibility.
The rate of the change caused by electromigration is roughly proportional to the diffusion coefficient of a material and current density. The self-diffusion coefficient of aluminum is approximately 2×10−23 m2/s at 200° C. according to the article of H. Mehrer, Landolt-Bornstein New Series III/26 (Springer-Verlag, 1990), and the diffusion coefficient of foreign atoms in aluminum does not exceed ten times of the self-diffusion coefficient of aluminum. Since the diffusion coefficient at 200° C. is approximately ten times larger than that at 80° C., the void generation time at a current density of 1×1010 A/m2, which was employed at the time of writing in the above Japanese Patent Laid-Open Publication No. Hei 08-293585, can be estimated to be approximately 1×107 s by use of the experimental value (a void formation time at a temperature of 80° C. and a current density of 1×109 A/m2 is approximately 1×109 s) obtained by S. Vaidya, et al. (Appl. Phys. Lett. 36, 464 (1980)). Assuming that only a very small amount of Si precipitation (a layer thickness of 1 nm= 1/1,000 of a thickness of a void formation region of 1,000 nm) is required and the diffusion coefficient of Si is ten times larger, the writing rate can be estimated to be 1,000 s/bit. Therefore, if the structure and the method disclosed in Japanese Patent Laid-Open Publication No. Hei 08-293585 are merely employed, the required time for writing is 15 minutes per bit, clearly showing impracticality.
A feasible technique for a device utilizing electromigration has been disclosed in Japanese Patent Laid-Open Publication No. 2001-267513. In this case, to solve the above problems, an alloy which has not been employed in an LSI has been employed, and an electrode for detecting atomic segregation is provided. The above publication also discloses that the temperature of the device itself is caused to increase or the device is caused to melt through the Joule heat generated by the electric current fed to the device to thereby allow electromigration to take place at a fast rate.
However, an increase in the rate of the change only by the increase in the device temperature is limited. If the device is caused to melt, the rate of the change significantly increases. However, the reaction with a contacting material is difficult to control, and thus the choice of an electrode material is strongly restricted. Therefore, there is an increasing demand for developing a material in which segregation takes place at a sufficiently fast rate and re-writing is facilitated without melting the device. However, such a material has not been proposed in Japanese Patent Laid-Open Publication No. 2001-267513.